Here it’s more useful to be clear about what can be said than to give an answer shaped like the question.
There’s no specific mattress to recommend. Mattress pads, along with pillows, foam cushions and booties, are things to use to reduce the pressure, and some pads are water- or air-filled to help support and cushion the area. A pad goes on a bed. It is not the bed.
The choice depends on the wound itself, and on whether the person is in bed or in a wheelchair. Talk with your health care provider about what choices would be best for you, including what shapes and types of material. No particular mattress or type is named as the right one or the wrong one.
That’s worth saying plainly, because specialist mattresses and overlays are expensive and sold with confident claims. There’s no general answer on which of them is best. The people who can answer that are the ones who can see the wound and the bed.
What is specific is everything that happens on top of whatever surface is there. Repositioning: if you are in bed, you should move or be moved about every 2 hours. Friction: avoid slipping or sliding as you move positions, and try to avoid positions that put pressure on your sore. And one bedding instruction with a condition attached: lightly dust your bedsheet with medicated talc powder to prevent further friction between your skin and the sheet — but only if your pressure sore is not an open wound.
A surface helps. It is never the whole answer.